The invention relates to the optometric examination of the human eye; more particularly, it relates to a method for automatically bringing the eye under examination and the eye examination apparatus into alignment with one another during the examination.
Apparatus such as objective refractors are commonly used to determine visual disorders of the eye and to prescribe corrective lenses. In using a typical objective refractor the patient sits before the apparatus and peers at a target, usually with his or her head held in approximately fixed position by a chin rest or other positioning member. For accurate diagnosis of refractive errors of the eye and for accurate prescription of corrective lenses, it is necessary that the eye be positioned in precise alignment with respect to the optical system of the objective refractor, and this position of alignment must be maintained while the diagnosis is being made.
A common problem faced by objective refractors is the initial acquisition of the eye under examination by the instrument's optical system and the maintenance of alignment while the visual disorders of the eye are measured. In many of the known objective refractors the instrument must be manually aligned with the eye under examination. Manual adjustment by an operator is at best of limited accuracy. For an operator who performs many examinations per day the need to manually align the instrument and eye, and to maintain that alignment during measurement, is inconvenient and annoying, with the result that the alignment is sometimes not properly carried out.
An objective refractor providing for some measure of automatic alignment of the eye is available from Humphrey Instruments, Inc. of San Leandro, Calif., and is disclosed in U.S. patent application Ser. No. 406,607, filed Aug. 9, 1982, and entitled "Objective Refractor for the Eye." The Humphrey instrument includes a projection system which casts a light pattern onto the retina of the eye under examination and which automatically analyzes the light returned from the retina. To use the Humphrey instrument, an operator first makes an initial manual adjustment of the alignment to bring the eye within the acquisition range, i.e., within the range of automatic machine alignment. The instrument includes a highly complex optical detector head which senses refractive errors in the light returned from the retina. The head is sufficiently sensitive to certain changes in the returned light pattern characteristic of lateral misalignment of the eye that it can provide a servo signal for automatic correction of the lateral misalignment. Axial misalignment can also be made to produce characteristic changes in the returned light pattern, but these were heretofore believed to be difficult to detect, difficult to correct reliably by automatic means, or at the least require sufficiently complex instrumentation that completely automatic alignment of the eye was deemed to be beyond the realm of commercial feasibility. Even though the instrument of application Ser. No. 406,607 is able to provide accurate and automatic correction of lateral misalignments, and is even able to sense to some extent the presence of axial misalignments, the designers of that commercially successful instrument still believed the intervention of a human operator was necessary to correct the axial misalignment manually as a separate operation.